Fremont Micro Devices

FT838NB/FT838RNA

FUNCTIONAL DESCRIPTION

Operating Description

FT838NB/FT838RNA is a cost effective and high-performance AC-DC power supply controller for off-line low

power AC-DC applications including battery chargers, adaptors and LED lighting. The constant voltage (CV)

and constant current (CC) control are achieved accurately without the secondary feedback circuit.

Start up Control

Start-up current of FT838NB is very low so that a start-up resistor with high resistance and low-wattage is

allowed to supply the start-up power for the controller. The large value startup resistor minimizes the power

loss in operations and allows quick start up. FT838RNA increases the start-up current slightly to ensure the

smooth shutting down operations in LED lighting applications with relaxed requirement on external board

design.

Operating current

The operating current of FT838NB/FT838RNA is as low as 500uA. Good efficiency is achieved with the low

operating current. Low operating current also reduces the Vcc hold-up capacitance requirement.

Constant voltage (CV) and constant current (CC) Operation

The FT838NB/FT838RNA can accurately achieve CV/CC characteristic output without secondary side

voltage and current-feedback circuits. It operates in CV mode to regulate the output voltage by capturing the

auxiliary winding feedback voltage at FB pin. The auxiliary winding feedback voltage is proportional to

secondary winding, so it provides controller the feedback signal from secondary side and achieves

constant-voltage output. In CC mode, the controller detects the secondary discharger peak current and the

discharger time, which determines the off-time of the base driver to make the output average current

constant.

Primary peak current modulation

Primary peak current is constant at constant current mode. And primary peak current is modulated at constant

voltage mode for the purposes of good dynamic load response and no audible noise over entire operating

range. According to the output loading current the current threshold voltage is modulated from 0.55V to 0.27V.

The current threshold voltage is 0.55V at rated load and 0.27V at light load.

Leading edge blanking

Each time the power NPN transistor is switched on, a turn-on spike occurs at the sense resistor. To avoid

premature termination of the switching pulse, a 400ns leading edge blanking time is built in. Conventional RC

filtering can therefore be omitted. During this blanking period, the current limit comparator is disabling and

cannot switch off the base driver.

Under voltage lockout (UVLO)

FT838NB/FT838RNA turn-on Vcc_on and turn-off Vcc_off are 17.6 V and 6.6 V, respectively. During start-up,

the hold-up capacitor must to be charged to 17.6V through the start-up resistor. The hold-up capacitor

continues to supply Vcc until power can be delivered from the auxiliary winding of the transformer. Vcc must

not drop below 6.6 V during this start-up process. This UVLO hysteresis window ensures that hold-up

capacitor is sufficient to supply Vcc during start-up.

© 2012 Fremont Micro Devices Inc.

Confidential Rev. 0.8

DS838NB/838RNA-A-page7